---
title: Query Helpers
---

The `@nestjs-query/core` package provides a number of helper functions to transform or apply queries to a list of items.

An example use case for these helpers would be to write a `QueryService` that wraps a store that does not support the
query options natively (e.g. An in memory collection of objects such as a static array of objects).

All examples will be based on the following DTO definition.

```ts
interface TestDTO {
  first?: string | null;

  last?: string | null;

  age?: number | null;

  isVerified?: boolean | null;

  created?: Date | null;
}
```

## applyFilter

The `applyFilter` helper applies a `Filter` to a single object or an array of objects.

### Arguments

- `dto: DTO|DTO[]`
  - If a single object a function that will test the dto against the filter, returning `true` when if it matches the
    filter.
  - If an array of objects is provided the array will be filtered returning a new array with all elements that match
    the filter.
- `filter: Filter<DTO>` - The filter to check the object[s] against. See [`Filtering`](../concepts/queries.mdx#filtering)

### Example

```ts
import { applyFilter } from `@nestjs-query/core`;

const dtos: TestDTO[] = [
  {first: 'Bob', last: 'Yukon'}
  {first: 'Alice', last: 'Yukon'}
  {first: 'Sally', last: 'Yukon'}
  {first: 'Zane', last: 'Yukon'}
]
const filtered = applyFilter(dtos, {first: {in: ['Bob', 'Sally']})
```

```ts
const dto: TestDTO = {first: 'Bob', last: 'Yukon'};
applyFilter<TestDTO>(dto, {first: {in: ['Bob', 'Sally']}) // true
applyFilter<TestDTO>(dto, {first: {eq: ['Alice', 'Zane']}) // false
```

## applySort

The `applySort` will sort an array of dtos.

:::info
Because `applySort` uses the native `Array#sort` method it may not exactly match the ordering you would expect from a
database.
:::

:::warning
It is expected that your data types all match. For example if you have a number field that also has some numbers
represented as strings the applySort method may not work as expected.
:::

### Arguments

- `dto: DTO[]` - The array of DTOs to sort.
- `sortFields: SortField<DTO>[]` - The sorting criteria. See [`Sorting`](../concepts/queries.mdx#sorting)

### Example

```ts
import { applySort, SortDirection, SortNulls } from `@nestjs-query/core`;

const dtos: TestDTO[] = [
  {first: 'Bob', last: 'Yukon'}
  {first: 'Alice', last: 'Yukon'}
  {first: null, last: 'Yukon'}
  {first: 'Sally', last: 'Yukon'}
  {last: 'Yukon'}
  {first: 'Zane', last: 'Yukon'}
]
const sorted = applySort(dtos, [
   { field: 'first', direction: SortDirection.DESC, nulls: SortNulls.NULLS_LAST },
])
```

The resulting sorted array would be.

```ts
[
  {first: 'Zane', last: 'Yukon'}
  {first: 'Sally', last: 'Yukon'}
  {first: 'Bob', last: 'Yukon'}
  {first: 'Alice', last: 'Yukon'}
  {first: null, last: 'Yukon'}
  {last: 'Yukon'}
]
```

## applyPaging

The `applyPaging` method will apply a `limit` and/or `offset` to an array of dtos.

### Arguments

- `dto: DTO[]` - The array of DTOs to page.
- `paging: Paging` - The paging arguments to apply. See [`Paging`](../concepts/queries.mdx#paging)

### Example

```ts
import { applyPaging } from `@nestjs-query/core`;
const dtos: TestDTO[] = [
  {first: 'Bob', last: 'Yukon'}
  {first: 'Alice', last: 'Yukon'}
  {first: 'Sally', last: 'Yukon'}
  {first: 'Zane', last: 'Yukon'}
]
const paged = applyPaging(dtos, {offset: 1, limit: 2})
```

The resulting paged dtos would be.

```ts
[
  {first: 'Alice', last: 'Yukon'}
  {first: 'Sally', last: 'Yukon'}
]
```

## applyQuery

The `applyQuery` uses the `applyFilter`, `applySorting`, and `applyPaging` methods to apply a `Query` to an array of
DTOs.

### Arguments

- `dto: DTO[]` - The array of DTOs to page.
- `query: Query<DTO>` - The query to apply to the array of dtos. See [`Queries`](../concepts/queries.mdx)

### Example

```ts
import { applyQuery, SortDirection } from `@nestjs-query/core`;

const dtos: TestDTO[] = [
  {first: 'Bob', last: 'Yukon', isVerified: true}
  {first: 'Alice', last: 'Yukon', isVerified: false}
  {first: 'Sally', last: 'Yukon', isVerified: true}
  {first: 'Zane', last: 'Yukon', isVerified: true}
]
const queryResult = applyQuery(dtos, {
  filter: { isVerified: { is: true } },
  sorting: [{ field: 'first', direction: SortDirection.DESC }],
  paging: { offset: 1, limit: 2 }
})
```

The resulting array of dtos would be.

```ts
[
  {first: 'Sally', last: 'Yukon', isVerified: true}
  {first: 'Bob', last: 'Yukon', isVerified: true}
]
```

## transformFilter

The transformFilter is used to remap fields in a `Filter`. This method is commonly used when defining a custom
[Assembler](../concepts/advanced/assemblers.mdx).

### Arguments

- `filter: Filter<From>` - The filter you want to transform.
- `fieldMap: QueryFieldMap<From, To>` - A map of fields where the key is a key in the From type, and the value is a
  key in the to type.

### Example

```ts
import { transformFilter, QueryFieldMap, Filter } from `@nestjs-query/core`;

class TestEntity {
  firstName!: string;

  lastName!: string;
}

const fieldMap: QueryFieldMap<TestDTO, TestEntity> = {
  first: 'firstName',
  last: 'lastName',
};

const dtoFilter: Filter<TestDTO> = {
  first: { eq: 'foo' },
  last: { neq: 'bar' },
};

const transformed = transformFilter(dtoFilter, fieldMap);
```

The new filter would be

```ts
{
  firstName: { eq: 'foo' },
  lastName: { neq: 'bar' },
}
```

## transformSort

The `transformSort` is used to remap fields in an array of `SortField<DTO>[]`. This method is commonly used when
defining a custom [Assembler](../concepts/advanced/assemblers.mdx).

### Arguments

- `sortFields: SortField<From>[]` - The array of sorting criteria to transform.
- `fieldMap: QueryFieldMap<From, To>` - A map of fields where the key is a key in the From type, and the value is a key in the to type.

### Example

```ts
import { transformSort, QueryFieldMap, SortField, SortDirection } from `@nestjs-query/core`;

class TestEntity {
  firstName!: string;

  lastName!: string;
}

const fieldMap: QueryFieldMap<TestDTO, TestEntity> = {
  first: 'firstName',
  last: 'lastName',
};
const dtoSort: SortField<TestDTO>[] = [
  { field: 'first', direction: SortDirection.DESC },
  { field: 'last', direction: SortDirection.ASC },
];

const transformed = transformSort(dtoSort, fieldMap);
```

```ts
[
  { field: 'firstName', direction: SortDirection.DESC },
  { field: 'lastName', direction: SortDirection.ASC },
];
```

## transformQuery

The `transformQuery` method uses the `transformFilter` and `transformSort` methods to remap a `Query`. This method is
commonly used when defining a custom [Assembler](../concepts/advanced/assemblers.mdx).

### Arguments

- `sortFields: Query<From>` - The query to transform.
- `fieldMap: QueryFieldMap<From, To>` - A map of fields where the key is a key in the From type, and the value is a key in the to type.

### Example

```ts
import { transformQuery, QueryFieldMap, Query, SortDirection } from `@nestjs-query/core`;

class TestEntity {
  firstName!: string;

  lastName!: string;
}

const fieldMap: QueryFieldMap<TestDTO, TestEntity> = {
  first: 'firstName',
  last: 'lastName',
};
const dtoQuery: Query<TestDTO> = {
  filter: { first: { eq: 'foo' }, last: { neq: 'bar' } }
  sorting: [
    { field: 'first', direction: SortDirection.DESC },
    { field: 'last', direction: SortDirection.ASC },
  ]
};

const transformed =  transformQuery(dtoQuery, fieldMap);
```

The resulting query would be.

```ts
{
  filter: { firstName: { eq: 'foo' }, lastName: { neq: 'bar' } }
  sorting: [
    { field: 'firstName', direction: SortDirection.DESC },
    { field: 'lastName', direction: SortDirection.ASC },
  ]
}
```

## getFilterComparisons

Used to search a filter get a list of comparison objects for a given key.

### Arguments

- `filter: Filter<DTO>` - The filter to search.
- `key: keyof DTO` - The key in the DTO object to search for in the filter object.

### Example

```ts
import { Filter, getFilterComparisons } from `@nestjs-query/core`;

class TestDTO {
  age!: number;

  title!: string;
}

const filter: Filter<TestDTO> = {
  age: { gte: 10 },
  or: [{ title: { like: '%bar' } }, { title: { eq: 'foobar' } }],
};

const comparisons = getFilterComparisons(filter, 'title');
```

The resulting array would be

```ts
[{ like: '%bar' }, { eq: 'foobar' }];
```

## getFilterOmitting

Used to get a filter with a given key removed.

### Arguments

- `filter: Filter<DTO>` - The filter containing the unwanted key.
- `key: keyof DTO` - The key in the DTO object to remove in the filter object.

### Example

```ts
import { Filter, getFilterOmitting } from `@nestjs-query/core`;

class TestDTO {
  age!: number;

  title!: string;
}

const filter: Filter<TestDTO> = {
  age: { gte: 10 },
  or: [{ title: { like: '%bar' } }, { title: { eq: 'foobar' } }],
};

const filterWithoutTitle = getFilterOmitting(filter, 'title');
```

The resulting filter would be

```ts
{
  age: { gte: 10 },
}
```
